A full charge should be around 25.00v to 25.50v. When the bdi displays 0% it should be around 23.80v to 24.10v. Make sure to measure the voltage at the battery connector before the main contactor to get the most accurate reading.
At a full charge each individual battery should sit around 6.35v. At 0% they should all be around 6.02v.
If the individual batteries drop below 6v that is a sign there is an issue with one or more of the batteries.
If the battery voltage is not matching with the bdi, then there is a voltage drop somewhere in the system and you may need to start testing components. Fuses and contactor are the best places to start.